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394         ON  THE CHANGE  OF  REFRANGIBILITY  OF  LIGHT.
the displacements, the vibrations would be isochronous, and could only he excited by ethereal vibrations having almost exactly the same period, but would be powerfully excited by such. According, in a solution of chlorophyll the dispersion comes on very suddenly; a large part of it is produced by active light of nearly the same refrangibility as the dispersed light; and the latter, by whatever active light produced, has nearly the same refrangibility that it had at first. This supposition, combined with the preceding theory, accounts also for the transparency of the fluid with respect to rays of less refrangibility than the first absorption band, for the great intensity of that band, for the rapidity with which opacity comes on at its less refrangible border, and the comparatively slow resumption of transparency on the other side. A difference of the same nature <w opposite sides of a maximum of opacity seems to be a very common phenomenon in absorption. On the other hand, in those numerous cases in which the dispersion comes on gradually, in the manner described in Art. 44, we may suppose the part of the forces of restitution depending on first powers of the displacements to be but small.
235. It may appear at first sight to be a formidable, objection to the theory here brought forward, that, in the experiment mentioned in Art. 216, the intensity of the disper.Mul light did not appear to be more than doubled when the intensity of the incident disturbance was doubled; and that, in the experiment described in Art. 215, the rays of low refrangibility did not appear to exercise any protecting influenee. But tin1. diHieulty may, I think, be got over by a very reasonable supposition. It seems very natural to suppose that a given molecule remains for the greater part of the time at rest, or nearly so, ami only now and then gets involved in vibrations. On this supposition, it is only a very small per-centage of the* molecules that, at. a «jivou instant are vibrating to an extent worth considering, (loneeive now a'stream of light consisting of the highly refrangible rays to be incident on a sensitive medium, and to cause I per cent,, of the sensitive molecules to vibrate considerably, the rest vibrating so little that they may be regarded as at rest. Now imagine a second stream, similar in all respects to the first, to influence the medium which is already under the influence of the first stream.